Schweitzer Engineering Laboratories
Solving Line Protection Challenges with Transient-Based Relays
Fault current characteristics continue to change as more wind-powered generators and inverter-based sources are added to the electric power grid. The system fault response is not only different than in the days of large synchronous generators, but it also varies based on the source design and the utility grid code. We have three ways to tackle the rising protection challenges: fine-tune the present protective relays, enforce a better fault response of the sources, and use protection principles that are less dependent on the sources. This article shares our experience with transient-based line protection and shows how it helps solve today’s line protection challenges.
Speed has always been a key aspect of protection performance. Fast tripping minimizes equipment damage and risk to the system, personnel, and public.
Today’s time-domain and traveling-wave protective relays operate in 1 to 2 ms…about an order of magnitude faster than their predecessors. Characteristics of sources, CT saturation, and series compensation have little or no impact on the security.
Ideally, we want a protection element to respond based on a fault data window that is as short as possible (speed) and as rich in information as possible (selectivity). Traditionally, the longer the fault data window, the more information it contains.
Therefore, we typically work with a tradeoff between protection speed and selectivity. This is not necessarily so when unconventional sources dominate the fault current.
Unconventional sources challenge today’s phasor-based line protection elements. The key problems are related to low fault current and low inertia and affect directional and distance elements, faulted-phase identification, and remote backup protection. Waiting for the fault phasors to stabilize or adding an intentional time delay does not necessarily increase the amount of fault information available to the relay. To the contrary, the longer the protection scheme waits, the less clear the situation may become as the control algorithms of the sources respond to the fault conditions.